1. Field of the Invention
The present invention relates to a multi-layer record carrier, such as, a recordable optical disk of the write-once or rewritable type, suitable to be scanned by a single scanning device and provided with at least two substantially parallel information layers, wherein data is written in units of blocks on tracks of the at least two information layers. Furthermore, the present invention relates to a method and a recording apparatus for recording data on such a record carrier, and a method of manufacturing such a record carrier.
2. Description of the Related Art
Optical data storage systems, such as optical disk drives, allow a storage of large quantities of data on an optical medium. The data is accessed by focusing a laser beam onto the recording layer of the medium and then detecting the reflected light beam. In reversible or rewritable phase-change systems, optical media with two stable phases is used. A data bit is stored on the media by converting a small local area to one stable phase. The data bit can be erased by reverting the written area back to the starting phase. The starting phase is typically a crystalline phase and the laser beam writes data by locally converting the material in the data layer to a stable amorphous phase. This can be achieved by heating the crystalline region above its melting point and then cooling it quickly so that the disordered structure gets fixed in place, resulting in an amorphous structure. The data bit can later be erased by converting the amorphous phase back to the starting crystalline phase. This is done when the amorphous region is heated and maintained at or above its crystallization temperature, or alternatively melted and slowly cooled until the region is crystallized. The data in this type of phase change system is read or detected as a change in reflectivity between a crystalline region and an amorphous region on the optical medium.
To increase the storage capacity of an optical disk, multiple recording layer systems have been proposed. An optical disk having two or more recording layers may be accessed at different spatially separated recording layers by changing the focal position of a lens. The laser beam is transmitted through the nearer recording layer to read and write data on the farther recording layer or layers. Multiple recording layer disks require that the intermediate recording layers between the disk surface onto which the laser light is incident and the last or farthest recording layer from that surface be light-transmissive. To maximize the disk storage capacity of such optical disks, the recording density is substantially constant across the entire disk surface.
In (rewritable) optical recording with random access, the data is usually written in units of ECC blocks (e.g., in CLV systems without headers), in fixed recording unit blocks of a fixed fraction of an ECC block, such as, for example, 2 Kbytes or 4 Kbytes of user data (e.g., in Zoned Constant Angular Velocity systems with headers where the distance between two headers is an integer multiple of these recording unit blocks), or in variable length fractions of an ECC block (e.g., in Digital Video Recording systems where the ECC block size is not an integer multiple of the distance between two headers, and writing is “simply” stopped before a header and restarted after a header with the inclusion of some segment run-in and segment run-out data to guarantee proper behavior of the electronics). These fractions of ECC blocks are called “Recording Frames” in DVR systems and “SYNC Frames” in DVD systems. In optical record carriers with headers, the record carrier is subdivided in sectors, each sector comprising a header containing an address uniquely identifying the sector and a recording unit block to which user data, preferably protected by an error detection and correction code (ECC), is recorded.
In DVR systems, a Zoned Constant Angular Velocity (ZCAV) system is used. In such systems, the capacity of a sector is not constant over the disk. The linear density is approximately constant and the number of tracks per zone is constant, but the length of a track (i.e., one circumference of the disc) is increasing with a factor of 2.4 from inner to outer radius of the disc, while the number of headers per revolution is constant. Thus, the number of bits between two headers is increasing. The DVR system and format is described in T. Narahara et al, “Optical Disc system for Digital Video Recording”, Techn. Digest ISOM/ODS (MD1) Jul. 11–15, 1999, Kauai Hi., SPIE Vol. 3864 (1999), 50–52 and Jpn. J. Appl. Phys. 39 Pt. 1 No. 2B (2000), 912–919, and in K. Schep et al, “Format description and evaluation of the 22.5 GB DVR disc”, Techn. Digest ISOM 2000 (September 2000).
When data is written, the newly written data has to be linked with the data that is already present in a controlled way to guarantee the validity of both the already present data as well as the newly written data. For example, when writing a new block behind an already written block, two measures are taken. First, the new block should not be written over the user data in the already present block. This is guaranteed by introducing a gap between the end of the present data block and the start of the new data block. Second, the new block should be read correctly, i.e., the reading electronics should have the ability to resettle, for example, the amplitude (by a gain control function) and the frequency and the phase (by a Phase Locked Loop for data detection). This is guaranteed by preceding the new data with a preamble field which contains a repeated pattern long enough for the circuitry to stabilize and be well settled before the first user data is read.
FIG. 6 shows a principle recording pattern of this general method wherein a recording unit block (i.e., a physical cluster) always ends with a postamble (PoA), i.e., a specific pattern to signal the end of user data (a kind of synchronization pattern), a guard field, i.e., a field containing dummy data to overwrite possible present old data which could confuse read-back of the newly written data such as, for example, the PoA of a previous recording, and a gap or gap portion to guarantee that the data of a possible present next cluster is not overwritten. The next recording unit block (i.e., the next physical cluster) starts with a gap, again to prevent overwriting, a guard field G1 and a preamble PrA. The area comprising the postamble PoA, the guard fields G1 and G2, the gaps and the preamble PrA is called the data linking area used for linking succeeding recording unit blocks (i.e., succeeding physical clusters).
The sinusoidal waveform indicated in FIG. 6 indicates a wobble signal recorded on the optical disk and used as a timing reference for deriving the write clock and as a position reference indicating the writing position. Predetermined maximum tolerances D1, D2 and D3 are allowed with respect to the start of the postamble PoA, and the end and start of the guard fields G1 and G2, respectively.
In dual or multi-layer systems, the above linking method leads to the problem that the lower layer is read while a significant area of the laser beam passes through the gaps of the upper layer or layers. Thus, the transmission characteristic or the degree of transmissivity of the upper layer differs in dependence on the gaps since the transmissivity of the upper layer is different in the written and in the non-written state. This problem increases when the gaps are large, such as, for example, in DVR systems where the gaps may have a length of typically about 150 μm while the diameter of the beam in the upper layer is about 40 μm when reading the lower layer and when the gaps are at the same angular position in neighboring tracks, such as, for example, on the radial areas of a CLV or ZCAV system where an integer number of ECC blocks fits almost exactly on an integer multiple of one or several circumferences.
Furthermore, in dual or multi-layer phase change disks, the effective power in the deepest (second) layer differs according to the physical structure of the upper (first) layer. For example, the transmission characteristics or the transmissivity of the header area and of the rewritable land/groove area of a phase change disk are different. The effect of the header area in the upper layer on the deepest layer is the sum of two effects. First, the headers have a transmission characteristic which may differ from that of the land/groove area. When no data is recorded in the land/groove area, this difference is, in general, relatively small. However, when data is recorded in the land/groove area, the difference may be substantial. Secondly, just before and just after the header area, the groove is not yet written with phase change data due to the fact that in a DVR system, the segment run-in starts and the segment run-out ends with a gap before the data is actually written. This gap is used for, for example, the random start position shift (in the segment run-in) used for increasing the number of overwrite cycles in a phase change disk and as reserved space (in the segment run-out) when, for example, the bit length used is slightly longer than the nominal length due to inaccuracies when deriving the write clock from the wobble signal or when writing with a (non-locked) crystal clock. Moreover, after and before these gaps, a guard field and a preamble field PrA is written, e.g., to allow settlement of the electronics.
Thus, the transmission problem due to the gap portion in the upper layer occurs between data clusters and between a data cluster and a header area.
The transmission problem due to the header area is significantly increased in case of a (random) misalignment between the header spokes of the upper and of the lower information layers. Header alignment or correspondence requires a tight centering and angular alignment between the two layers. E.g., in the DVR system, less than approximately one wobble, i.e., less than 30 μm peak-to-peak, would be a preferred range for the displacement between the headers in the two information or recording layers. This displacement may result from unroundness (assumed to be small, i.e., less than a few μm), eccentricity (de-centering of the center of the spiral track with respect to the central hole) and angular difference. The de-centering of the spiral track with respect to the central hole is introduced mainly in the molding step of the disk mastering and replication process. The above-mentioned preferred allowed displacement range between the header corresponds to an allowed eccentricity of 15 μm peak-to-peak and an allowed angular misalignment of 0.015 degree.
U.S. Pat. No. 5,715,225 discloses a multi-layer optical record carrier and data storage device, wherein two separate optical heads are used to improve the alignment between the different information layers. Each optical head includes its own actuator. By using two separate actuators mechanically and electronically linked, tracking and sector information can be obtained from the appropriate recording layer of the optical storage device which was preformed or formatted. Thus, one of the optical heads is continuously focused on a recording layer which contains tracking and format information. This recording layer is not necessarily the recording layer on which user data is being recorded or read. Furthermore, U.S. Pat. No. 5,764,620 discloses a multi-layer optical record carrier in which the recording layers have sensitivity peaks in different wavelength, while allowing lights having the other wavelength to transmit therethrough. Each track on the recording layers is divided into a plurality of sectors, each of which having an identification section. The identification sections are shifted against one another in the tracking direction to thereby prevent the light beam from illuminating neighboring identification sections simultaneously. As a result, cross-talk or influences between neighboring identification sections can be reduced.